1. Field of the Invention
The invention relates to an exhaust gas recirculation catalyst and a process of producing the exhaust gas purifying catalyst.
2. Description of the Related Art
There has been known an exhaust gas purifying catalyst comprising a layer of a mixture of alumina powder supporting ceria and ceria powder coated on a catalyst support member and one noble metal selected from a group of platinum (Pt), rhodium (Rh) and palladium (Pd) supported by the layer and a process of making the exhaust gas purifying catalyst. Such an exhaust gas purifying catalyst and a process making the same are disclosed in, for example, Japanese Unexamined Patent Publication No. 60-110335 and Japanese Patent Publication No. 2-15253. The exhaust gas purifying catalyst has been made with an intention to prevent a reduction in specific surface area of alumina powder and to provide improvement of dispersibility of a noble metal on the alumina powder by reducing an amount of ceria supported directly on the alumina powder.
It has also known in the art to utilize an oxygen (O.sub.2) storage effect to expand the A/F window of an three-way catalyst. The term "A/F window" means an optimum range of air-fuel (A/F) ratio on either side of the stoichiometric value for engine operation. The above mentioned publications teach that the amount of ceria supported on ceria is reduced in view of preventing a reduction in specific surface area of an alumina powder and a ceria powder is added after supporting a noble metal on an alumina powder with an effect of ensuring a sufficient amount of ceria necessary for the catalyst to provide a desired oxygen storage effect.
While the prior art teaches that the exhaust gas purifying catalyst is effective to prevent a reduction in specific surface area of an alumina powder, however, there is not any suggestion for improving an interaction between palladium oxide and cerium oxide. In order to enhance catalytic function of the palladium oxide with an oxygen storage effect of the cerium oxide, it is necessary that the cerium oxide is suitably dispersed around the palladium oxide. However, simply mixing a ceria powder with an alumina powder after supporting a noble metal on the alumina powder does not secure it that a noble metal is dispersed in close proximity to ceria powder so as to cause cerium oxide to present a desired oxygen storage effect.